Thermoplastic resins are formed into many kinds of articles, for example by extrusion, thermoforming, and compression molding processes. The thermoplastic resins are usually compounded and manufactured as pellets or powders that may be easily shipped and that may be easily and conveniently handled during formation of the desired articles or stored for later processing. It is most economical to manufacture the thermoplastic resins as pellets in a continuous extrusion process in which the pellets are compounded to meet the requirements of many uses.
It is often the case, however, that a thermoplastic resin must be specially formulated or customized for a particular use or application, such as mold-in color processes, for example by including special additives for that use, such as pigments to achieve a particular color or a stabilizer package that meets the requirements of a particular use. In particular, thermoplastic resins may be customized to include certain pigments and/or colorants. It is often desirable that the manufactured article have a particular color. A particular color may, for example, enhance aesthetic appeal of the article or may even serve to help identify the particular brand or manufacturer.
Customizing the color or additive package of thermoplastic resins can, however, present problems. While the general purpose thermoplastic resin, or base resin, may be produced by continuous extrusion operations, colored thermoplastic resins or thermoplastic resins with special additive packages are typically required in much smaller amounts that are relatively expensive to produce. Manufacture, especially post-production cleaning, is extremely labor instensive. The processing equipment (such as blenders, feeders, extruders, and pelletizing equipment) must be thoroughly cleaned after each particular color or customized blend in order to avoid contamination of the thermoplastic resins that will next be produced in the equipment. The cleaning process requires significant down time of the equipment during which no material is being manufactured, adding to the manufacturing costs of the specialized product.
Alternatively, colored articles have been produced by dry blending the uncolored thermoplastic resin with a color concentrate (also known as color masterbatch) in what is known in the art as a "salt and pepper blend." The thermoplastic resin and the color concentrate used for such blends are typically of similar size pellets or pieces. The color concentrate is usually a minor amount by weight of the blend, typically only up to about 5% by weight, and often much less. The salt and pepper blend is then introduced directly to the molding or forming equipment used to produce the final article. The melt blending of the uncolored resin and the color concentrate must take place in the molding or forming equipment. This process, however, can result in color variations from piece to piece, or even in areas within the same piece, because of incomplete blending, particularly for low blending ratios of the color concentrate. Segregation of particular additives during packaging and transportation may also be experienced with dry blends and contribute to lack of homogeneity in the final formed articles.
In a method related to the use of a salt and pepper blend, lakes and liquid colors can be introduced at the mouth of an injection molding extruder or into other forming equipment. This method also has a number of drawbacks. In the first place, sophisticated metering equipment may be required by the fabricator in order to produce uniform coloration. In the second place, this method is similar to the salt and pepper method in that the coloring material and the uncolored resin are introduced essentially separately, relying on a thorough mixing in the forming equipment before the article is finally shaped. Incomplete blending and color variations can easily result.
Gose et al., in U.S. Pat. No. 5,443,910 and related patents, describe applying certain processing, stabilizing, or other functional polymer additives to polymeric particles by spraying the particles with an aqueous emulsion of the polymer additive. The emulsion includes an emulsifiable (acid-functional) wax, surfactant, a base, the additive, and the water. This method of customizing thermoplastic resins, however, has several drawbacks. First, the inclusion of a base compound in the applied material may cause unwanted coloration or color shifts. For example, Gose et al. note a problems when potassium hydroxide or sodium hydroxide is used. It is also well-known that discoloration may result from amines in certain systems. Secondly, the water of the emulsion, or the combination of water and base, present problems for hydrophilic resins. In addition, it may be desirable in some instances to exclude surfactants from the thermoplastic resin. Finally, the emulsion application method has drawbacks that are inherent in the process, for instance, that the applied layer of emulsion must be dried and the emulsion must be stored and used under carefully maintained conditions to prevent destabilization and separation.
Sharma, in U.S. Pat. No. 5,300,256 and related patents, also describes applying an additive to a polymeric particle from an aqueous medium. Sharma discusses the handling and storage problems of the Gose and provides a solid two-phase additive system that can be dispersed in water just prior to application. The polymer additives used by Sharma must melt at a temperature no higher than 100.degree. C., and may include hydrophilic polymers such as low molecular weight polyolefins with carboxylic moieties. The additives again contain water and are further diluted with water and applied as an aqueous emulsion, still with many of the attendant problems as mentioned above.
It is also known to prepare color concentrates by a method of blending together a mixture of pigment, low molecular weight polyethylene wax, and thermoplastic polyolefin granules. The pigment and wax is included at relatively high levels, for example approximately 40% by weight of the mixture. The wax dispersed the pigment forms a layer on the polymer granules. This masterbatch is blended with unmodified base polyolefins to form a "salt and pepper" blend. The high loading of pigment and dispersing wax is required by the masterbatch in order to achieve the desired final pigment loading in the salt and pepper blend. Such masterbatch-containing blends suffer from the same problems discussed above. In addition, the relatively thick coating of wax on the granules is undesirable for the reason that such thick coatings tend to easily chip and flake off, which could result in segregation and nonhomogeneity in the molded articles, as well as dust problems in production.
Thus, it would be desirable to provide a customized thermoplastic resins that overcomes the drawbacks of the those produced by the methods described above.
We have now discovered a novel method of preparing customized thermoplastic resins in which the pieces, typically pellets, of thermoplastic resin are coated with a coating that comprises a polymeric component and an additive component. The polymeric component of the coating has a melting point, softening point, or glass transition temperature that is lower than the melting point, softening point, or glass transition temperature of the thermoplastic resin body. The above-mentioned melting point, softening point, or glass transition temperature (or other temperature or temperature range where the resin begins to soften or flow) of a particular resin or resin composition may conveniently be termed the onset temperature of the melt processing range. It will be appreciated by the skilled artisan that the term "onset temperature for the melt processing range" is used herein for convenience, and that it may refer to either a single distinct temperature or to a range of temperatures. In referring to differences between ranges, one may employ the difference of the highest, lowest, or average temperature within a range; the term preferably employs the most meaningful separation of temperature in the application under consideration.
In a preferred embodiment, the coating is a color concentrate comprising a polymeric component and one or more pigments, optionally with other additives. In a process for producing the customized thermoplastic resins of the invention, the thermoplastic resin body or piece is contacted with the coating at a temperature at which the coating composition is a melt. The coated resin pieces or pellets are cooled to solidify the coating in a layer on the thermoplastic resin body. In a preferred embodiment, a substantially even thickness of an outer layer of the customizing coating is applied. The coated resin pieces or pellets are easy to handle and may be formed into articles using the same processes and in the same way as would resin pellets that have had the color or other additive integrally mixed in.
The color concentrate or other customizing coating composition is coated onto the resin after the resin piece is formed. The customized thermoplastic resins of the invention may thus be produced in easy-to-clean equipment and without requiring down time of the equipment used to manufacture the thermoplastic resin body, resulting in a much more versatile and economical process. In addition, because each pellet or other body of the thermoplastic resin is in intimate contact with the pigment or additive necessary to produce the desired customized resin, defects such as inhomogeneity, which are especially apparent when color is involved, are avoided. Further, the surfactants, water, amines, and resins with hydrophilic functionalities required by the prior art compositions are not necessary for the coatings of the present invention.
In particular, the present invention provides for a customized thermoplastic resin that has a thermoplastic resin body and an outer thermoplastic coating layer, covering at least a portion of the body, comprising a polymeric component and an additive component. It is particularly preferred that the additive is a pigment.